There have been generally-known imaging devices such as camera-equipped portable telephone devices and digital still cameras that use approximately 3- or 5-million pixel solid-state imaging elements such as CCDs or CMOSs, and have imaging lenses of an aperture Fno of approximately 2.8 mounted thereon.
Such imaging devices are now required to be even smaller in size, and the imaging lenses mounted on those imaging devices are required to have smaller sizes and shorter total optical lengths than ever.
In recent years, in small-sized imaging devices such as camera-equipped portable telephone devices, imaging elements have become smaller and become capable of coping with a larger number of pixels. Models with high-pixel imaging elements almost equivalent to digital still cameras have become popular. Therefore, the imaging lenses mounted on small-sized imaging devices are required to have high lens performance suitable for such high-pixel solid-state imaging devices.
Further, there is a demand for lenses with brighter aperture Fno, to prevent image quality degradation due to noise caused when an image is captured in a dark place. Each of such small-sized and high-performance imaging lenses needs to have a four-lens structure or a structure with more lenses. (see Patent Documents 1 through 5, for example)